This invention relates to a method of increasing the conductivity of ground ferrophosphorus particles. In particular, it relates to a process wherein ground ferrophosphorus particles are treated by immersion in a solution of hypophosphorous acid.
Due to concern about the corrosion of automobile bodies, the automotive industry has increasingly used galvanized steel for making auto body parts. While galvanized steel offers increased protection from corrosion, it is more difficult to form and weld than is uncoated steel. In spot welding galvanized steel, for example, the copper electrodes used in the welding can alloy with the zinc coating on the steel to form brass. The brass has a lower melting point, and is weaker than the steel, and bonds to the electrode. This results in portions of the electrode breaking off, which significantly reduces the life of the electrode.
The formation of brass at the tip of the electrode can be greatly reduced by coating the galvanized steel with a primer containing ferrophosphorus particles in a resinous binder. (see, for example, U.S. Pat. Nos. 4,794,050, 3,884,705 and 4,109,763). In order for the welding currents to effect the weld, it must pass through the ferrophosphorus coating. This means that there must be a sufficient number of conductive ferrophosphorus particles in the coating, and their points of contact must be conductive enough to initiate the weld.
Ferrophosphorus is a co-product in the production of phosphorus. To obtain the ferrophosphorus pigment particles used in these coatings, the ferrophosphorus is ground to the desired particle size. The degree to which the grinding operation reduces the particle size of the ferrophosphorus influences the efficacy of the ferrophosphorus-containing weldable primer. The optimum particle size is determined by the thickness of the primer applied and the desired function of the primer. For example, the particles must be smaller than the coating thickness or rough or dull surfaces will result, yet not so small that a large number of particle to particle contacts are required to achieve the conductivity essential for weldability. Each interface represents a point of minimum conductivity and too many such points can hurt weldability. The ferrophosphorus-containing weldable primer can serve several functions in addition to corrosion protection; simple weldability alone is obtained with low (about 15% by volume) loadings of the ferrophosphorus pigment while weldability at lower welding currents and with extended electrode life is achieved with higher loadings (about 45% by volume). Of course, the primer must be formulated to satisfy manufacturing, application and economic considerations. It is apparent that increased conductivity at the ferrophosphorus particle surface would enable a wider ranger of particle sizes and pigment loadings to be used, thus facilitating formulation and allowing the use of primers superior in performance or economics to those now available.
It has further been found that the conductivity of ground ferrophosphorus particles can be increased by treatment with an acid, such as hydrochloric, nitric, or sulfuric acid. Although not yet fully understood, it is likely that acidic treatment of ground ferrophosphorus dissolves oxides and/or phosphates on the particle surface formed during the grinding operation and/or removes surface contaminants introduced during processing. However, this increased conductivity is transitory and the resistance of the particles increases rapidly after a short period of time. An improved treatment of ferrophosphorus particles to increase conductivity is described in U.S. Pat. No. 4,728,462. In that patent, the particles are treated with an aqueous solution containing a dilute mineral acid and ions of chromium and a metal selected from the group consisting of zinc, strontium, calcium, barium, lead, or copper. While that treatment is effective in increasing the conductivity of the particles, the use of chromium in the treatment creates environmental problems in the manufacture of the primer and in the welding of the primed metal.